1. Field of the invention
The present invention concerns a fuel injection method for diesel internal combustion engines using main injection of a gas main fuel and pilot injection of a liquid secondary fuel which initializes combustion of the main fuel. This is known as dual fuel injection.
2. Description of the prior art
The prior art injection method for dual fuel diesel engines which operate with two fuels, one a gas and the other a liquid, is to initialize the combustion of the main fuel (the gas) by injecting a liquid fuel representing 5 to 10% of the maximum volume that can be injected by the injector pump. The injector pump is designed to enable the engine to achieve its maximum power from the liquid fuel only, in the temporary absence of the gas fuel.
The minimum volume of liquid fuel injected is limited by the geometrical precision with which the injector pump and injector components are manufactured. This precision determines the lower limit below which it is no longer possible to guarantee a balance between the cylinders and the regular injection of a sufficient volume to initialize combustion (approximately equivalent to 2% of the maximum energy input). The minimum volume of liquid fuel injected is further limited by the dimensions of the holes of the injector atomizer which are optimized for correct atomization at full engine power. They are then too large to atomize properly a few percent of the maximum flowrate.
Experience shows that reducing the volume of the pilot injection reduces exhaust carbon monoxide and nitrogen oxides. What is more, the high cost of the liquid fuel is an incentive to reducing its use.
The present invention proposes to reduce the liquid fuel pilot injection volume to the thermodynamically sufficient minimum independently of the precision with which the injector pump is manufactured and independently of the dimensions of the atomizer holes. This can be achieved by mixing water with the liquid fuel.